1. Technical Field of the Invention
This invention relates generally linear motor fluid pumps and compressors and specifically to linear air pumps and controlling the pressure and airflow produced by linear air pumps.
2. Background Art
A linear electric motor directly produces a reciprocating motion suitable for driving pumps and compressors. The motor has a spring loaded armature that is constrained to move substantially in a single linear direction. One or more drive coils are positioned so that when an electric current passes through the coil, the magnetic field generated exerts a force on the armature causing it to move in the linear direction. The armature has a natural resonance frequency determined by the mass of the armature and the spring constant. When the coil is driven by an alternating electrical current at the resonance frequency, the linear motor efficiently produces a continuous reciprocating motion.
Linear motors are commonly used to drive diaphragm air pumps, especially for low pressure applications such as aeration of aquariums, fish ponds, and chemical processing tanks, and for inflating objects such as rafts, advertising displays, and air mattresses.
The pressure produced by a diaphragm pump depends on the airflow, reaching a maximum pressure when there is no airflow. For example when a pump is used to inflate an object with a finite volume, initially the airflow is high and the pressure is low. As the object fills, the pressure increases as the airflow decreases until airflow become zero when the object is fully inflated. The pump produces maximum pressure when there is zero airflow. The mechanical characteristics of the motor and pump determine exactly how the pressure varies as a function of the airflow.
In many applications, the pressure produced by pump must be regulated to optimize a process or prevent damage to the object or system connected to the pump. Pressure regulation can be accomplished by varying the current to the drive coils or by maintaining constant drive power while using a valve mechanism to vary the airflow and/or air pressure.
Compared to using valves, varying the drive current to control the pressure provides the advantages of using less power and increasing the operational life of the pump. This is especially important for applications that require a wide dynamic range of airflows and a pressure limit substantially less than the pump's maximum pressure. An example of such an application is the multi-zone control system for residential forced-air HVAC systems disclosed in U.S. Pat. No. 6,983,889 issued Jan. 6, 2006 to Alles. This system uses inflatable bladders to independently control the airflow to each vent. Maximum airflow is required as many bladders are being inflated or deflated, but the airflow approaches zero as all the bladders become fully inflated or deflated. However, the pump must continue to supply pressure to keep the bladders inflated and provide airflow to compensate for inevitable small leaks.
The method of using periodic pulse width modulated DC current to control the power produced by a linear pump is understood by those ordinarily skilled in the art. Such pumps are readily available form several suppliers. For example, the 6000 series DC pumps supplied by Thomas Product Division, 1419 Illinois Ave., Sheboygan, Wis. 53081, USA uses a variable pulse-width drive to control the power. The pump provides an input connection that can be externally driven to control the power, and therefore control the airflow and pressure produced by the pump.
When a periodic DC current pulse is used to drive the coil, there is a period of time when the coil is not driven. The drive circuit is in a high impedance state during this period. The collapsing magnetic field of the coil and the motion of the armature produce a back EMF (electro motive force) signal across the coil. Some linear motors use a magnetized armature to increase the coupling between the coil and the armature, thus increasing the efficiency of the motor. A magnetized armature produces a back EMF signal that is more dependent on the armature motion.
U.S. Pat. No. 6,437,524 issued Aug. 20, 2002 to Dimanstein discloses a drive circuit for a linear motor that uses the back EMF signal as feedback to adjust the frequency of the drive signal to match the natural resonance frequency of the motor and compressor system to increase power and improve efficiency.
Rotating electronically commutated motors (ECM) typically use back EMF signals to control the switching from one drive coil to another. In general the frequency of the drive pulses controls the speed and the pulse widths control the torque. U.S. Pat. No. 6,307,336 issued Oct. 23, 2001 to Goff et al. disclosed a closed loop control system where the back EMF provides feedback to the circuits that drive the coils.
The prior art has developed controls that provide specific relations between speed and torque. For example, The ThinkTank 3.0 motor supplied by GE ECM by Regal-Beloit Corporation, 200 State Street, Belait, Wis. 53511 provides a programmable ECM for blowers used in the HVAC industry. The back EMF signal provides feedback to the control circuits so the torque and speed can be independently controlled. An integrated microprocessor can be programmed to produce a specific relation between torque and speed. For example when attached to a blower, the speed and torque can be programmed to maintain a constant airflow over a wide range in pressures.
Control methods based on back EMF have not been adapted for applications that use linear motors such as air pumps. While variable power linear motors are available, external sensors and circuits are required to produce the signal that controls the power. These external components increase the cost of the system and reduce its reliability